In order to create a faster and more reliable means of transporting information, a plurality of communication links connecting a common pair of endpoints can be aggregated. This is known as “link aggregation”. Link aggregation, or IEEE 802.3ad, is a computer networking term which describes using multiple network cables/ports in parallel to increase the link speed beyond the limits of any one single cable or port, and to increase the redundancy for higher availability.
In digital communications, information is divided into groups of bits called packets. One prior art method of link aggregation is to create a single, virtual, communication link between a pair of endpoints. By virtue of the aggregation, the virtual communication link has more desirable properties than any of the constituent communication links being aggregated. For example, the virtual link can have higher transmission capacity, higher reliability, etc.
Different aggregation policies can be used to construct virtual links, in accordance with desired performance objectives. Generally there are performance tradeoffs involved in selecting a suitable aggregation policy. For example, suppose the performance objective is to maximize the aggregate transmission capacity, or throughput, subject to the constraint of the virtual link serving packets in First-In First-Out (FIFO) order. If the latency suffered by packets on a communication link is variable, then packets may arrive at one endpoint out of order. Thus, a reordering buffer at the remote endpoint is necessary to store packets that arrive out of order until they can leave the virtual link and be delivered in order to the client at the remote endpoint. As the load on the communication links increases, the variability of latency may increase, thereby increasing the delay required to reorder packets at the remote endpoint, and hence increasing the end-to-end latency of the virtual link. Thus there is a tradeoff between maximum throughput of the virtual link and the maximum latency of the virtual link.
Real-time communications applications may not tolerate such a large latency, and an aggregation policy for the virtual link aimed at maximizing throughput may not be appropriate for such situations. Alternatively, the aggregation policy could be aimed at minimizing latency. For example, one possible approach is to replicate each packet at the local endpoint and send a copy of each packet on each communication link. The purpose of such redundant transmissions is to exploit the fastest communication link. Specifically, the first packet of the set of replicated packets to arrive at the remote endpoint is delivered to the client, and redundant packets subsequently received are discarded at the remote endpoint. If the offered load is sufficiently low, this aggregation policy would yield lower latency than an aggregation policy aimed at maximizing throughput. On the other hand, with this alternate aggregation policy, the maximum throughput is reduced, due to the use of the redundant packet transmissions.
If the set of applications supported on a virtual link have similar requirements, an appropriate aggregation policy may be easily identified. However, if there are some applications to be supported whose requirements (like high throughput) conflict with other applications with different requirements (like low latency), a sacrifice or compromise must be made in selecting an appropriate aggregation policy. For example, if the link aggregation policy used is aimed at maximizing throughput, then latency sensitive applications may suffer from long latency. Conversely, if the link aggregation policy used employs transmission redundancy to reduce latency, then throughput sensitive applications may have limited throughput.
In view of the forgoing, there is a need for a method of link aggregation that can more flexibly deal with supported applications that have heterogeneous performance requirements.